Hello,
I developed and implemented an environment identical to what you are describing and have a known working recipe that you could easily replicate for your own system.
This environment is for a calling card and SIP wholesale service in Asia. It consists of a centralized A2Billing Database with redundant/synchronized DB backup, single A2B GUI admin interface, front-ended by multiple Asterisk servers, each architect-ed to handle ~200 calls each. Some of the Asterisk servers interface directly with the local Telco using E1 interfaces, others connect to commercial SIP trunk providers. Everything is redundant, fail-safe and easily scalable by simply adding more Asterisk resources. I was able to do all of this without the need of a SIP proxy front end but if you're requirements call for that, it is simple to incorporate one into my design.
I have all of this already spec'd out with Visio diagrams, BOMs for servers, firewalls, etc. I even created a rapid install routine to quickly deploy new Asterisk nodes.
My proposal includes this "recipe" which I will deliver to you consisting of all of the specifications and documentation that I have MONTHS invested in, along with 2-3 hours of remote consultation to help you understand the nuts and bolts of my proposal and otherwise guide you in the right direction for your particular needs.
Thank you for your consideration.